Microwave landing systems (MLS) facilitate aircraft approach and landing operations. These systems include so-called precision distance measuring equipment (DME/P) which provides aircraft distance information by measuring total round-trip time between pulse interrogations from an airborne transmitter and replies from a ground transponder. This type of system provides high accuracy ranging information in the severe multipath environment encountered during landing operations. In operation, the ground transponder in the DME/P generates a time delay upon receipt of an interrogation pulse. After a fixed time, a reply pulse is transmitted by the transponder through use of an RF transmitter.
To provide accurate aircraft distance information over the closely-shaped channels in the system (usually located 1 MHz apart), the RF pulse transmitter in the transponder must provide a carefully shaped reply pulse to help conserve the available spectrum. Generation of precisely-shaped RF pulses is usually accomplished by high level collector voltage modulation of RF power amplifier transistors. Such transistors are typically biased in a Class C mode of operation. This mode results in large gain variations of the amplifier over the dynamic range of the DME/P pulse, and therefore the task of stabilizing feedback-controlled modulators with which the amplifier is used is greatly complicated. Moreover, the abrupt turn-on characteristics of Class C amplifiers also necessitate use of driver pulses with "pedestals," which complicates the modulator design and adjustment process.
There is therefore a need for an improved RF pulse modulated amplifier for use in a pulse transmitter of a DME/P, which overcomes these and other problems of the prior art.